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Comment by nine_k | Hacker News Reader

Quite likely they have a good approximate number.

But knowing the exact number can indeed be hard. It would take stopping the entire uploading and deletion activity. Of course they may have counters of uploads and deletions on every node which handles them, but the notion of 'the same instant' is tricky in distributed systems, so the exact number still remains elusive.

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It’s not just tricky and elusive, I think it’s literally unknowable -- not a well-defined question. Like asking about the simultaneity of disconnected events in special relativity.

You can modify the question to be well-defined and not suffer those measurement problems, eg "the total number of videos uploaded before midnight UTC on 2021-04-29"

Interesting, I wonder if a distributed database could be developed to consistently answer queries phrased in this way.

Seems like it would either need to be append only or have some kind of snapshot isolation

Google Developed Spanner which is a globally distributed database that uses atomic clocks to keep things synchronized: https://static.googleusercontent.com/media/research.google.c...

I think the Chandy-Lamport snapshot algorithm tries to do something like this for all distributed systems (in their model, and it tries to get any consistent snapshot, not allowing you to specify the "time"); not sure if it's actually useful IRL though

All these nodes are in the same light cone, so we theoretically can stop all mutation and wait for the exact final state to converge to a precise number of uploaded videos.

But the question of the precise number of videos before that moment is indeed ill-defined.

You can theoretically stop all mutation, but the users might start to complain!

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